Balanced angular accelerometer

ABSTRACT

A balanced angular accelerometer is provided having a substrate, a fixed electrode with a plurality of fixed capacitive plates, and a rotational inertia mass with a central opening and substantially suspended over a cavity and including a plurality of movable capacitive plates arranged to provide a capacitive coupling with the first plurality of fixed capacitive plates. The accelerometer has a central member and an outer member fixed to the substrate. According to one embodiment, a plurality of inner support arms extend between the central member and the inertia mass and a plurality of outer support arms extend between the inertia mass and the outer member to support the mass over the cavity. According to another embodiment, one or more cut out apertures are formed in the inertia mass to compensate for a channel and signal line so as to balance the inertia mass about the center of the inertia mass.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is related to application Ser. No. 09/782,708entitled “ANGULAR ACCELEROMETER,” filed on Feb. 13, 2001. Theaforementioned related application is incorporated herein by reference.

[0002] This application is one of two applications filed on the samedate, both commonly assigned and having similar specifications anddrawings, the other application being identified as U.S. ApplicationSerial No. [Docket No. DP-307174], entitled “ANGULAR ACCELEROMETERHAVING BALANCED INERTIA MASS.”

TECHNICAL FIELD

[0003] The present invention generally relates to angular accelerometers(i.e., rotational acceleration sensors) and, more particularly, to abalanced microfabricated angular accelerometer.

BACKGROUND OF THE INVENTION

[0004] Accelerometers are commonly employed to measure the secondderivative of displacement with respect to time. In particular, angularaccelerometers measure angular acceleration about a sensing axis.Angular accelerometers are frequently employed to generate an outputsignal (e.g., voltage) proportional to the sensed angular accelerationfor use in vehicle control systems. For example, the sensed accelerationsignal may be used to determine a potential vehicle rollover event andto control automotive devices in response thereto. Angularaccelerometers may also be used to control a disk drive read/write headsuch that a control system associated therewith may compensate forsevere shock and/or vibrations that cause the angular acceleration.

[0005] One approach to determining angular acceleration employs anangular velocity sensor to sense angular velocity, and differentiatesthe sensed angular velocity to determine the angular acceleration. Thedesign for an angular velocity sensor is generally complex, and angularvelocity sensors are typically expensive to produce. In addition,acceleration measuring devices employing an angular velocity sensortypically require a differentiator which adds to the complexity andoverall cost of the device.

[0006] Another approach for determining angular acceleration uses acombination of two linear accelerometers mounted to a rigid body forsensing linear acceleration along two respective perpendicular axes.Generally, the linear accelerometers each employ a mass suspended from aframe by multiple beams. The mass, beams, and frame act as a spring-masssystem, such that the displacement of the mass is proportional to thelinear acceleration applied to the frame. The signal extracted from twolinear accelerometers can be used to extract angular accelerationinformation. Linear accelerometers are readily available and easy touse; however, in order to measure angular acceleration while rejectinglinear acceleration, the scale factor, i.e., sensitivity or gain, of thetwo sensors generally must be matched.

[0007] A further approach for an angular accelerometer is disclosed inU.S. Pat. No. 5,251,484, entitled “ROTATIONAL ACCELEROMETER,” whichemploys a circular hub centrally supported on a substrate and connectedto radially disposed thin film spoke electrodes that flex in response toangular acceleration. Rotational acceleration measurement is achieved byusing a differential, parallel plate capacitive pick-off scheme in whichthe flexible spoke electrodes at the periphery of the fixed disk rotatebetween fixed reference electrodes so that an off-center position ofmoving electrodes results in a measured differential voltage from whichthe disk motion is determined. The sensing capability for such anaccelerometer is generally limited to the amount of movement of theflexible spoke electrodes. This cantilevered design with rotaryelectrodes generally requires high structural matching to ensurepredictable gain, phase, and linearity response. The linear andcross-axis sensitivity (gain) is highly dependent on the structuralmatching. Additionally, separate input and output contacts for eachcapacitive plate add to the overall complexity and cost of theaccelerometer.

[0008] More recent designs of angular accelerometers are disclosed inU.S. application Ser. No. 09/410,712, filed on Oct. 1, 1999, and U.S.application Ser. No. 09/782,708, filed on Feb. 13, 2001, both assignedto the assignee of the present application. The microfabricated angularaccelerometers disclosed in the aforementioned U.S. patent applicationshave a rotational inertial mass formed on a substrate and suspended overa cavity via a plurality of support arm tethers. Such accelerometersachieve enhanced sensitivity over previously known accelerometers.However, the design of some angular accelerometers may result in poorlinear cross-axis sensitivity on at least one axis, particularly foraccelerometers having an asymmetric structure.

[0009] Accordingly, many conventional angular accelerometers oftensuffer from various drawbacks including errors introduced by cross-axisaccelerations. It is therefore desirable to provide for a low-cost, easyto make and use, enhanced sensitivity angular accelerometer thateliminates or reduces the drawbacks of the prior known angularacceleration sensing devices, including enhancing the sensitivity of thesensor to structural asymmetries, fabrication processing, packaging,impulsive shocks due to handling, and temperature-induced stresses.

SUMMARY OF THE INVENTION

[0010] In accordance with the teachings of the present invention, anangular accelerometer is provided. The angular accelerometer includes asubstrate, a fixed electrode supported on the substrate and having afirst plurality of fixed capacitive plates, and a rotational inertiamass substantially suspended over a cavity and including a centralopening and a plurality of movable capacitive plates arranged to providea capactive coupling with the first plurality of fixed capacitiveplates. The angular accelerometer also includes a central member fixedto the substrate and located substantially in the central opening of therotational inertia mass, and an outer member supported on the substrateand located radially outward from the rotational inertia mass. A firstplurality of support arms extend between the central member and therotational inertia mass, and a second plurality of support arms extendbetween the rotational inertia mass and the outer member. The first andsecond plurality of support arms allow rotational movement of therotational inertia mass upon experiencing an angular acceleration.

[0011] In the disclosed embodiment, an input is electrically coupled toone of the fixed electrode and the rotational inertia mass for receivingan input signal, and an output is electrically coupled to the other ofthe fixed electrode and the rotational inertia mass for providing anoutput signal which varies as a function of change in the capacitivecoupling and is indicative of angular acceleration. According to oneaspect of the present invention, the angular accelerometer issubstantially symmetric about an axis to provide a balanced rotationalinertia mass. By connecting the rotational inertial mass to both thefixed central member and the outer member via the first and secondpluralities of support arms, the angular accelerometer minimizes linearand cross-axis sensitivities and provides a rugged sensing mechanicalstructure, while exhibiting a high sensing-axis response.

[0012] These and other features, advantages and objects of the presentinvention will be further understood and appreciated by those skilled inthe art by reference to the following specification, claims and appendeddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The present invention will now be described, by way of example,with reference to the accompanying drawings, in which:

[0014]FIG. 1 is a top view of an angular accelerometer formed on asubstrate according to a first embodiment of the present invention;

[0015]FIG. 2 is a cross-sectional view of the angular accelerometertaken through lines II-II of FIG. 1;

[0016]FIG. 3 is an enlarged view of section III of FIG. 1;

[0017]FIG. 4 is an enlarged view of section IV of FIG. 1;

[0018]FIG. 5 is a top view of the rotational inertia mass shown removedfrom the angular accelerometer of FIG. 1;

[0019]FIG. 6 is a top view of the central member and support arms shownremoved from the angular accelerometer of FIG. 1;

[0020]FIG. 7 is a block/circuit diagram illustrating processingcircuitry coupled to the angular accelerometer;

[0021]FIG. 8 is a top view of an angular accelerometer havingalternative outer support arms according to a second embodiment of thepresent invention;

[0022]FIG. 9 is a top view of an angular accelerometer having a balancedinertial mass according to a third embodiment of the present invention;

[0023]FIG. 10 is a cross-sectional view taken through lines X-X of FIG.9; and

[0024]FIG. 11 is a top view of an angular accelerometer having abalanced inertial mass and an alternative signal line according to afourth embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] Referring to FIGS. 1 and 2, an angular accelerometer 10 isillustrated according to a first embodiment of the present invention forsensing angular acceleration about the Z-axis extending perpendicular toa plane defined by the X-Y-axes. The angular accelerometer 10 sensesangular acceleration about the sensing Z-axis, while preventing thesensing of linear and angular off-axis accelerations along non-sensingaxes. The angular accelerometer 10 is a micromachined accelerometerhaving a rotational inertial mass and supporting structure whichminimizes the sensitivity of the microsensor to structural asymmetries,fabrication processing, packaging, impulse shocks due to handling, andtemperature-induced stresses. Additionally, the angular accelerometer 10has high sensitivity due to high mechanical gain, and thus is lesssensitive to noise interference such as electromagnetic interference(EMI).

[0026] The angular accelerometer 10 is fabricated on a single-crystalsilicon substrate 60 using a trench etching process. The trench etchingprocess may include etching out a pattern from a doped materialsuspended over a cavity 34 to form a conductive pattern that ispartially suspended over the cavity 34. One example of an etchingprocess that may be used to form the angular accelerometer 10 of thepresent invention is disclosed in commonly assigned application Ser. No.09/410,713, filed on Oct. 1, 1999, entitled “MEMS STRUCTURE ANDMICROFABRICATION PROCESS,” which is incorporated herein by reference.While the angular accelerometer 10, as described herein, is fabricatedon a single-crystal silicon substrate using a trench etching process, itshould be appreciated that the angular accelerometer 10 could befabricated using other known fabrication techniques, such as: an etchand undercut process; a deposition, pattern, and etch process; and anetch and release process, without departing from the teachings of thepresent invention.

[0027] The angular accelerometer 10 includes a rotational inertia mass12 suspended over cavity 34 above substrate 60. Rotational inertia mass12 is generally shown configured in the shape of a circular annular ringhaving a circular central opening in the center region. However, itshould be appreciated that rotational inertia mass 12 may be configuredin various other shapes and sizes without departing from the teachingsof the present invention. A stationary central member 15 istrench-etched from the mass 12 and is fixedly attached to the underlyingsubstrate 60 via oxide layer 64, centered within the circular opening ofthe rotational inertia mass 12. The rotational inertial mass 12 has aplurality of rigid comb-like conductive fingers 14 extending radiallyoutward from the outer peripheral edge to serve as movable capacitiveplates. The rotational inertia mass 12 with comb-like conductive fingers14, is a movable mass that is rotatable angularly about the Z-axis, whensubjected to an angular acceleration about the Z-axis. For purposes ofdiscussion herein, the X-Y plane is defined as the plane formed by theX-axis and the Y-axis as oriented in FIG. 1, while the Z-axis is definedas the axis which extends perpendicular to the X-Y plane as shown inFIG. 2.

[0028] The rotational inertia mass 12 is suspended above cavity 34 viafour inner support arms (i.e., tethers) 16A-16D connected to thestationary central member 15 and four outer support arms (i.e., tethers)40A-40D connected to a stationary outer member. According to theembodiment shown, the stationary outer member includes isolators 18 andoutput line 30 which are fixed to the substrate. Accordingly, therotational inertia mass 12 is supported both on the inside via the fixedcentral member 15 and on the outside via the fixed outer member, shownas isolators 18 and line 30. According to the first embodiment shown anddescribed herein, the four inner support arms 16A-16D are equiangularlyspaced from one another by ninety degrees (90°). In addition, the fourouter support arms 40A-40D are likewise equiangularly spaced from oneanother by ninety degrees (90°), and are angularly offset forty-fivedegrees (45°) relative to the orientation of the inner support arms16A-16D. While four inner support arms 16A-16D and four outer supportarms 40A-40D are shown and described herein, it should be appreciatedthat any number of a plurality of support arms may be employed for eachthe plurality of inner and outer support arms in accordance with theteachings of the present invention, such as eight, twelve, or sixteensupport arms. However, it is preferred, but not required, that theangular accelerometer 10 contain an even number of inner and outersupport arms.

[0029] The inner support arms 16A-16D are integrally formed as radialextensions connecting the annular ring-shaped inertia mass 12 to thefixed central member 15. A pair of parallel trench-etched slots(trenches) 17 are etched in the rotational inertial mass 12 to form eachof the inner radial support arms 16A-16D. The slots 17 extend throughthe entire depth of the inertial mass 12 and, in effect, result in slots17 formed on opposite sides of each of inner support arms 16A-16D. Theslots 17 extend from the inner edge defining the central opening in theinertial mass 12 radially outward to a location where the correspondingsupport arm is connected to the inertial mass 12. The slots 17 form airgaps which allow the inner support arms 16A-16D to be connected at alocation further radially outward from the inner edge, thereby providingfor an increased effective overall length and greater angularflexibility of the support arms 16A-16D.

[0030] The outer support arms 40A-40D are integrally formed asextensions connecting the annular ring-shaped inertia mass 12 to astationary outer member fixed to the substrate. The stationary outermember is shown including three isolators 18 and output signal line 30,all of which are fixed to the substrate. The three isolators 18 areconnected to support arms 40B-40D, while support arm 40A is connected tooutput signal line 30. It should be appreciated that the stationaryouter member may include other stationary members which are fixedrelative to the substrate. A pair of parallel trench-etched slots(trenches) 41 are etched in the inertia mass 12 to form each of theouter support arms 40A-40D. The slots 41 extend through the entire depthof the inertial mass 12 and, in effect, result in slots 41 formed onopposite sides of each outer support arm 40A-40D which allows the outersupport arms 40A-40D to be connected at a location radially inward fromthe outer perimeter, thereby providing for an increased effectiveoverall length and greater flexibility of the outer support arms40A-40D. According to the first embodiment shown, the outer support arms40A-40D each include a pair of folded semi-circular portions whichprovide an increased overall effective length, thus increasingflexibility and compliance, and reducing stiffness of the support arm.The outer support arm 40A, in addition to supporting the rotationalinertia mass 12, provides a conductive path for transmitting an outputelectrical signal from rotational inertia mass 12 and movable capacitiveplates 14 to output signal line 30.

[0031] The inner and outer support arms 16A-16D and 40A-40D,respectively, are flexible beams that act as springs which are compliantto bending perpendicular to the longitudinal axis of the beam in the X-Yplane, but are relatively stiff to bending out of the X-Y plane in thedirection of the Z-axis. The support arms 16A-16D and 40A-40D preferablyhave a thickness (depth) in the range of three to two hundredmicrometers, and a width in the range of one to twenty micrometers.According to one example, support arms 16A-16D and 40A-40D may have athickness of approximately thirty microns as compared to a width ofapproximately five microns to provide sufficient aspect ratio ofthickness-to-width to allow for flexibility in the X-Y plane andstiffness in the Z-axis.

[0032] Together, the four inner support arms 16A-16D and the four outersupport arms 40A-40D symmetrically suspend the rotational inertia mass12 above cavity 34 in the X-Y plane, and yet allow angular rotationabout the Z-axis when subjected to angular acceleration about theZ-axis. The rotational inertia mass 12 and support arms 16A-16D and40A-40D are symmetric with respect to an axis passing through the centerof the central member 15, and thus the angular accelerometer 10 iselectrically and physically balanced. By employing at least twoorthogonal pairs of opposing inner support arms 16A-16D and at least twoorthogonal pairs of opposing outer support arms 40A-40D, the entirestructure is symmetric and is stiff with respect to linear accelerationsin the X-Y plane. Yet, the rotational inertia mass 12 is free to rotatewith good sensitivity about the Z-axis within the constraints of theinner and outer radial support arms.

[0033] Fixed to a thick oxide insulation layer 64 on top of substrate 60are four fixed electrodes 20A-20D, each having a plurality of fixedcapacitive plates 24 interdisposed between adjacent movable capacitiveplates 14, to form four banks of variable capacitors. The first fixedelectrode 20A has a clock input line 22A for receiving a square waveclock signal CLKB 26. The plurality of fixed capacitive plates 24provided with the first fixed electrode 20A are interdisposed betweenadjacent movable capacitive plates 14 of rotational inertia mass 12 forapproximately one-quarter rotation (i.e., a ninety degree window) ofinertia mass 12, to provide a first bank of capacitors. The second fixedelectrode 20B likewise has a plurality of fixed comb-like capacitiveplates 24 interdisposed between adjacent movable capacitive plates 14 ofinertial mass 12 for approximately one-quarter of its rotation, toprovide a second bank of capacitors. The second fixed electrode 20B hasa clock input 22B for receiving a square wave clock signal CLK 28. Thethird fixed electrode 20C also includes a plurality of fixed comb-likecapacitive plates 24 for approximately one-quarter of movable capacitiveplates 14 of inertia mass 12, to provide a third bank of capacitors, andlikewise receives clock signal CLKB 26 via input line 22C. The fourthfixed electrode 20D has a plurality of fixed capacitive plates 24 forapproximately the remaining one-quarter of the movable capacitive plates14 of inertia mass 12, to provide a fourth bank of capacitors, andreceives clock signal CLK 28 via clock input line 22D. It should beappreciated that the number of fixed electrodes can be increased tomultiples of four, as represented by equation 4×N, where N=1, 2, 3, 4,etc., which may advantageously provide for good matching and cross-axisrejection.

[0034] Each of the fixed electrodes 20A-20D are formed near the outerperimeter of the rotational inertia mass 12 extending through an angularrotation of approximately ninety degrees (90°). Adjacent fixedelectrodes 20A-20D are dielectrically isolated from one another viatrenches 41 which form isolators 18 and output line 30. Each isolator 18has surrounding slots that serve to provide a dielectric air gap. Thefixed electrodes 20A-20D and corresponding plurality of fixed capacitiveplates 24 are fixed in place supported on top of insulation layer 64 andsubstrate 60. Additionally, each fixed electrode has an arcuateconductive strip formed on top thereof and connected to correspondinginput lines 22A-22D to enhance the signal transmission. The rotationalinertia mass 12 and its rigid outer peripheral capacitive plates 14 areable to move relative to fixed capacitive plates 24 in response to arotational acceleration experienced about the Z-axis.

[0035] The rotational inertia mass 12 and movable capacitive plates 14are electrically conductive and are electrically coupled to output pad32 via support arm 40A and output signal line 30 for providing an outputcharge V_(O). The output charge V_(O) is processed to determine avoltage indicative of the angular rotation of the rotational inertiamass 12 relative to the fixed electrodes 20A-20D due to angularacceleration about the Z-axis. Accordingly, by measuring the outputcharge V_(O) at output pad 32, the angular accelerometer 10 provides anindication of the angular acceleration experienced about the Z-axis.

[0036] With particular reference to the cross section shown in FIG. 2,the angular accelerometer 10 includes substrate 60 which serves as theunderlying support. Substrate 60 may include a silicon or silicon-basedsubstrate having the thick oxide insulation layer 64 formed on the topsurface, and a bottom oxide insulation layer 62 formed on the bottomsurface. The substrate 60 may include silicon, or alternative materialssuch as glass or stainless steel, for example. The substrate 60 andoxide insulation layer 64 are configured to provide a cavity 34 belowthe rotational inertia mass 12. Additionally, substrate 60 and oxidelayer 64 form a central pedestal 36 below the fixed central member 15for purposes of fixing the central member 15 in place relative to thesubstrate 60. Central pedestal 36 also provides structural supportduring the fabrication process.

[0037] Formed above the substrate 60 and on top of insulation layer 64is an EPI layer 66. EPI layer 66 is made of a conductive material and isetched to form various components including the rotational inertia mass12, central member 15, isolating trenches 80, air gaps 25, and otherelements that support or isolate conductive signal paths. Trenches 80and air gaps 25 provide physical and electrical isolation betweenadjacent elements. The EPI layer 66 may have a thickness in the range ofthree to two hundred micrometers. With the main exception of therotational inertia mass 12 and central member 15, the EPI layer 66further includes a field passivation layer 68 disposed on the topsurface thereof. The conductive signal paths of electrodes 20A-20D,lines 22A-22D, and data line 30 are formed on top of the dielectricfield passivation layer 68 as shown to provide signal transmissionpaths. In addition, a passivation layer 90 is formed over each of thesesignal paths.

[0038] It should be appreciated that the angular accelerometer 10 may beformed by disposing the EPI layer 66 and insulation field passivationlayer 68 on top of substrate 60. Prior to the etching process, thecentral pedestal 36 provides structural support for EPI layer 66 toallow the central member 15 to be fixedly provided on top thereof. Byproviding a central pedestal 36, the structural integrity of theaccelerometer 10 is enhanced during the fabrication process. After theetching process, the central pedestal 36 supports the central member 15which, in turn, partially supports the rotational inertia mass 12 viainner support arms 16A-16D. By supporting the EPI layer 66 in thecentral region during the manufacturing process, the maximum stressexperienced is greatly reduced. This allows the use of larger cavitysizes for a given thickness of EPI layer 66, resulting in greatersensitivity and signal-to-noise ratio.

[0039] Referring to FIG. 3, a portion of the angular accelerometer 10 isfurther illustrated in greater detail. Outer support arm 40A and signalline 30 extend within a pair of parallel radial slots 41 formed throughthe entire depth of rotational inertia mass 12 to provide an electricalpath between the rotational inertia mass 12 and output pad 32. The slots41 provide dielectric isolation between each of the data line 30 andsupport arm 40A and rotational inertial mass 12, as well as betweenadjacent fixed electrodes 20A and 20B while allowing the rotationalinertia mass 12 to rotate within limits imposed by the inner and outersupport arms. Trenches 80 isolate the fixed electrodes from the outersurrounding elements.

[0040] The fixed capacitive plates 24 are interdisposed between adjacentmovable capacitive plates 14 and separated from one another via an airgap 25. The air gap 25 between capacitive plates 14 and 24 allows formovable capacitive plates 14 to move relative to the fixed capacitiveplates 24. Each of the movable capacitive plates 14 has a very smallmass as compared to the rotational inertia mass 12, and are rigid toprevent rotary movement relative to rotational inertia mass 12.Additionally, the movable and fixed capacitive plates 14 and 24,respectively, each has a thickness equal to the thickness of the EPIlayer 66. Because total change in capacitance is proportional to thethickness of the capacitive plates 14 and 24, the signal-to-noise ratiois enhanced with enlarged thickness.

[0041] The air gap 25 between capacitive plates 14 and 24 is greater onone side of plate 14 as compared to the opposite side. For example, onthe bank of capacitors formed by electrode 20B, the width W_(L) of airgap 25 between capacitive plates 14 and 24 is approximately twice thewidth W_(S). The air gap 25 between adjacent pairs of capacitive plates14 and 24 is configured substantially the same for each of the fixedcapacitive plates 24 connected to the fixed electrode. However, foradjacent fixed electrodes 20A and 20B, the orientation of the capacitiveplates 14 and 24 is switched in that the larger air gap width W_(L) andsmaller gap width W_(S) of air gap 25 is on the opposite side ascompared to the adjacent fixed electrodes. For example, the fixedcapacitive plates 24 on fixed electrode 20A are separated from movablecapacitive plates 14 by an air gap 25 of width W_(L) twice as wide onthe left side of capacitive plates 14 as the width W_(S) on the rightside of capacitive plates 14, while fixed electrode 20B is configuredwith a larger air gap width W_(L) on the right side of capacitive plates14 as compared to its left side.

[0042] Additionally, each of the fixed capacitive plates 24 may includeenlarged motion stop beads 27 for limiting the relative movement betweencapacitive plates 14 and 24 in the event excessive angular accelerationis experienced. Motion stop beads 27 can be formed on either or both ofthe movable and fixed capacitive plates 14 and 24, respectively.

[0043] The angular accelerometer 10 is shown and described in connectionwith four banks of variable capacitors formed by capacitive plates 14and 24. The capacitive plates 24 associated with fixed electrodes 20Aand 20C have a certain positive-to-negative orientation with respect tomovable capacitive plates 14. In contrast, the positive-to-negativeorientation between capacitive plates 14 and 24 for the fixed electrodes20B and 20D are arranged oppositely of the adjacent fixed electrode. Byalternating the orientation of the plurality of four banks of capacitorsin the four equi-angular sections as disclosed, the angularaccelerometer 10 essentially nulls out any cross-axis acceleration andlinear acceleration, and allows for angular acceleration to be sensedabout the Z-axis. Further, by employing a plurality of fixed capacitiveplates 24 commonly connected to fixed electrodes 20A-20D, a reducednumber of signal input and output lines is achieved.

[0044] Referring to FIG. 4, another enlarged portion of the inertialmass 12 of angular accelerometer 10 is illustrated in greater detail.Each of the radial inner support arms 16A-16D is formed as a continuousconductive line which extends from the fixed central member 15 to therotational inertia mass 12 at a location displaced radially outward fromthe central member 15. Inner support arms 16A-16D each provide a tetherconnection between central member 15 and rotational inertia mass 12.Support arms 16A-16D are formed by etching to remove material to formthe bordering slots 17. Support arms 16A-16D flex within slots 17 toallow rotational movement of the rotational inertia mass 12 relative tothe central member 15. Accordingly, support arms 16A-16D provide rigidvertical support in the Z-axis, while allowing for angular rotationabout the vertical Z-axis.

[0045] Each of the outer support arms 40A-40D is likewise formed as acontinuous conductive line which extends from the stationary outermember, shown as isolators 18 and line 30 fixed to the substrate, to therotational inertia mass 12 at a location displaced radially inward fromthe outer peripheral edge thereof. Outer support arms 40A-40D eachprovide a tether connection between the fixed outer member and therotational inertia mass 12. It should be appreciated that outer supportarm 40A is formed of a continuous conductive signal line which, inaddition to physically supporting the rotational inertia mass 12, alsotransmits electrical signals to output line 30. Outer support arms40A-40D are formed by etching to remove material to form the borderingslots 41. Outer support arms 40A-40D flex within slots 41 to allowmovement of the rotational inertia mass 12 relative to the substrate.Accordingly, outer support arms 40A-40D also provide rigid verticalsupport in the Z-axis, while allowing for angular rotation of theinertia mass 12 about the vertical Z-axis.

[0046] The central member 15 is separated from the inner circular edgeat the central opening of ring-shaped rotational inertia mass 12 via airgap 13. Air gap 13 is formed as a set of arc-shaped slots betweenadjacent inner support arms 16A-16D by etching away material from theEPI layer forming inertia mass 12 and central member 15 through thecomplete depth to form a set of segmented circular slots having width ofpreferably at least the width W_(S). According to one example, air gap13 has a width of approximately five microns. The air gap 13 has a widthsufficiently large to allow the rotational inertia mass 12 to rotaterelative to the central member 15 without interference, yet is smallenough to allow for a large surface area of the ring-shaped inertia mass12.

[0047] The rotational inertia mass 12 is further shown in FIG. 5, withthe inner support arms 16A-16D, outer support arms 40A-40D, and centralmember 15 removed. Rotational inertia mass 12 includes slots 41, eachshown as a single slot, with the corresponding outer support arms40A-40D removed, formed through the entire depth and extending inwardfrom the outer perimeter for defining an opening in which the outersupport arms 40A-40D are disposed. In addition, radial slots 17 extendfrom the central opening defined by air gap 13 to a location radiallyoutward for providing an opening within which the corresponding innersupport arms 16A-16D are located and capable of flexing. The rotationalinertia mass 12 as shown is ring-shaped in that the central region has agenerally circular opening to receive central member 15 and air gap 13.

[0048] The inner support arms 16A-16D, outer support arms 40A-40D, andcentral member 15 are further illustrated in FIG. 6, removed from therotational inertia mass 12. As can be seen in FIGS. 5 and 6, the centralmember 15, inner support arms 16A-16D and outer support arms 40A-40D,fit within slot 13, slots 17, and slots 41, respectively, of therotational inertia mass 12. One end 84 of each of support arms 16A-16Dand 40A-40D is integrally attached to rotational inertia mass 12 at alocation 82 shown in FIG. 5.

[0049] Referring to FIG. 7, processing of the signals applied to andsensed with the angular accelerometer 10 is illustrated according to oneembodiment. The fixed electrodes 20A-20D are generally shown receivingclock signal CLKB at pad 26 and signal CLK at pad 28. Clock signals CLKBand CLK may be rectangular (e.g., square), wave-generated signals thathave alternating voltage levels of V_(S) and zero volts or +V_(S) and−V_(S). Clock signal CLKB is one hundred eighty degrees (180°) out ofphase, i.e., inverse, as compared to clock signal CLK, and thereforeprovides an opposite phase rectangular waveform. The processingcircuitry includes a summer 42 for receiving the output voltage V_(O) onpad 32 and a voltage V_(O2) received from the summation of thecapacitors, represented herein as CT, when a voltage source V_(S) isapplied thereto. Voltage V_(O2) contains externally induced noise (e.g.,EMI and/or RFI noise) present in the sensed signal, and summer 42subtracts the noise from the output charge V_(O). The output of summer42 is applied to a charge-to-voltage converter and demodulator 44 whichconverts the processed charge to a voltage signal. The voltage signal isthen input to a summer 46 which receives a signal from an offset trim48. The offset trim 48 provides a signal which compensates for bias andbias drift, including bias drift due to temperature variations.Accordingly, summer 46 sums the trim signal with the voltage output soas to compensate for bias errors. The bias compensated voltage is thenapplied to an output driver and gain trim 52 which rescales the voltageto within a desired range and produces the output signal V_(OUT). Itshould be appreciated that the output signal V_(OUT) may be furtherprocessed via further control circuitry, such as a microprocessor-basedcontroller, to perform various control functions.

[0050] In operation, the angular accelerometer 10 provides a measurementof the angular acceleration about the Z-axis, while being non-responsiveto cross-angular accelerations and linear accelerations. In doing so,the rotational inertia mass 12, when subjected to an angularacceleration about the Z-axis, rotates about the Z-axis relative to thefixed electrodes 20A-20D and within the restraining limits of thesupport arms 16A-16D and 20A-20D. If the rotational inertia mass 12 isrotated in a positive direction about the Z-axis, the opposing banks ofvariable capacitors formed by fixed electrodes 20A and 20C increase incapacitance, while the opposing banks of variable capacitors formed byelectrodes 20B and 20D decrease in value, or vice versa. The change incapacitance provides the output signal V_(O) indicative of the angularacceleration experienced. Since inner support arms 16A-16D and outersupport arms 40A-40D are integrally formed within slots 17 and 41,respectively, in the rotational inertia mass 12, and are attached to thefixed central member 15 and the outer member, susceptibility to damageby external shock is thus reduced.

[0051] Referring to FIG. 8, an angular accelerometer 10′ is shown havingouter radial support arms 40A′-40D′ according to a second embodiment ofthe present invention. In contrast to the outer support arms havingfolded semi-circular portions in the first embodiment, the outer radialsupport arms 40A′-40D′ of the second embodiment are formed as straightarms extending radially outward. The outer radial support arms 40A′-40D′are each bounded on opposite sides by straight slots 41′. Accordingly,the rotational inertia mass 12′ is symmetrically supported by straightradial inner and outer support arms 16A-16D and 40A′-40D′, respectively.While the inner support arms 16A-16D and outer support arms 40A-40D and40A′-40D′ have been shown and described herein in connection withstraight line and folded semi-circular configurations, it should beappreciated that the inner and outer support arms may be configured invarious sizes, shapes, and numbers, without departing from the teachingsof the present invention.

[0052] Referring to FIG. 9, an angular accelerometer 110 is shown havinga signal line 140 and cut out apertures 150 formed in a rotationalinertia mass 112 to achieve a centrally balanced inertia mass 112according to a third embodiment of the present invention. The angularaccelerometer 110 includes a rotational inertia mass 112 which isgenerally asymmetric, in contrast to the above-described first andsecond embodiments of angular accelerometers 10 and 10′. Angularaccelerometer 110 employs similar features described in connection withthe angular accelerometer 10, and thus identical reference numerals areused to identify identical features. It should also be appreciated thatthe angular accelerometer 110 may be manufactured according to thetechniques described above in connection with the manufacture of theangular accelerometer 10.

[0053] The angular accelerometer 110 employs a plurality of innersupport arms 16A-16H which connect the rotational inertia mass 112 tothe central member 15, as explained above. In addition, the angularaccelerometer 110 employs a conductive signal output line 140 extendingfrom the rotational inertia mass 112 to the output signal line 30. Theconductive signal line 140 integrally attaches to rotational inertiamass 112 at a location radially inward from the outer perimeter thereof.In this third embodiment of angular accelerometer 110, no furtherconductive signal lines or outer support arms are connected to the outermember or other peripheral members. Instead, the single conductive line140 conducts electrical signals from the rotational inertia mass 112 tothe output signal line 30. The conductive element 140 is formed similarto support arm 40A (FIG. 1) by forming slots 141 on opposite sides ofthe conductive element 140 so as to allow the conductive element 140 toflex during angular rotation of the rotational inertia mass 112.

[0054] It should be appreciated that the presence of a single conductiveelement 140 and bordering slots 141 results in an asymmetric rotationalinertia mass which, for a constant thickness inertia mass 112, cause animbalance of the rotational inertia mass 112 relative to the center ofcentral member 15. By removing material from mass 112 to form slots 141,a reduction in the weight of the mass 112 on one side is created, thusresulting in the imbalance. The presence of the imbalance created by theasymmetric design may result in reduced sensitivity to linear cross-axisaccelerations, at least for one of the axis.

[0055] The angular accelerometer 110, according to the third embodimentof the present invention, employs one or more cut out apertures 150formed in the rotational inertia mass 112 to balance the rotationalinertia mass to cause the center of mass of the rotational inertia mass112 to be substantially centered at the center of the rotational inertiamass 112. The cut out apertures 150 are formed by etching or otherwiseremoving material opposite the side of the slots 141 and conductivesignal line 140 and are sized to compensate for the imbalance created byforming slots 141 and conductive signal line 140 in the opposite side ofrotational inertia mass 112. Referring to FIG. 10, the plurality of cutout openings 150 are shown extending completely through the rotationalinertia mass 112. However, it should be appreciated that the cut outopenings may extend completely or partially within mass 112. It shouldalso be appreciated that the number of cut out apertures 150 may includeany number having a size sufficient to balance the rotational inertiabeam 112 to provide the center of mass about the center of the inertiamass 112. Additionally, the location of the cut outs apertures 150 ispreferably along an axis opposite the slots 141.

[0056] By providing mass balancing of the rotational inertia mass 112,an equalization of the frequencies of the orthogonal mode is realizedwhich, in turn, significantly improves the cross-axis responses of thestructures. The mass balancing also introduces a process benefit in thatthe cut out openings 150 in the rotational inertia mass 112 mayfacilitate cavity venting prior to the release of the fine structuralgeometries. This further prevents excessive finger motions and hence thecomb-like fingers from structural damage during a venting event.

[0057] Referring to FIG. 11, an angular accelerometer 110′ is shownhaving a radial conductive element 140′ and cut out apertures 150according to a fourth embodiment of the present invention. Conductiveelement 140′ is shown as a radially straight line extending from thecentral member 15 radially outward to signal line 30. The conductiveelement 140′ is formed by removing material on opposite sides to formstraight radial slots 141′. The cut out apertures 150 are formed toremove a sufficient amount of the mass opposite of the conductiveelement 140′ so as to balance the rotational inertia mass 112′ about thecenter of the rotational inertia mass 112′ which is at the center ofcentral member 15.

[0058] By connecting the rotational inertia mass 12 to the fixed centralmember 15 via the plurality of inner support arms 16A-16D, (FIG. 1) andfurther connecting the rotational inertia mass 12 to the fixed outermember via the outer support arms 40A-40D, the angular accelerometer 10is less sensitive to stresses induced by fabrication processing,packaging, handling, and structural asymmetries. By providing cut outapertures 150 (see FIGS. 10 and 11) in the rotational inertia mass tocompensate for an imbalance in the rotational inertia mass, the angularaccelerometer 112 is provided in the balanced state and, thus, is lesssensitive to linear cross-axis accelerations. Additionally, therealization of high gain enhances immunity to EMI signals andenvironmental conditions such as humidity and temperature. Further, theangular accelerometer provides high gain for angular accelerations aboutthe sensing axis, while minimizing linear and cross-axis sensitivities.The resultant angular accelerometer achieves low sensitivity to externalhandling and environmentally induced stresses, and can be manufacturedat low cost.

[0059] It will be understood by those who practice the invention andthose skilled in the art, that various modifications and improvementsmay be made to the invention without departing from the spirit of thedisclosed concept. The scope of protection afforded is to be determinedby the claims and by the breadth of interpretation allowed by law.

1. An angular accelerometer comprising: a substrate; a fixed electrodesupported on the substrate and including a first plurality of fixedcapacitive plates; a rotational inertia mass substantially suspendedover a cavity and including a central opening and a plurality of movablecapacitive plates arranged to provide a capacitive coupling with saidfirst plurality of fixed capacitive plates; a central member fixed tosaid substrate and located substantially in the central opening of saidrotational inertia mass; an outer member supported on the substrate andlocated radially outward from the rotational inertia mass; a firstplurality of support arms extending between said central member and saidrotational inertia mass; and a second plurality of support armsextending between the rotational inertia mass and the outer member,wherein the first and second plurality of support arms allow rotationalmovement of the rotational inertia mass upon experiencing an angularacceleration about a sensing axis.
 2. The angular accelerometer asdefined in claim 1 further comprising: an input electrically coupled toone of the fixed electrode and the rotational inertia mass for receivingan input signal; and an output electrically coupled to the other of thefixed electrode and the rotational inertia mass for providing an outputsignal which varies as a function of change in the capacitive couplingand is indicative of sensed angular acceleration.
 3. The angularaccelerometer as defined in claim 2, wherein one of said input andoutput is electrically coupled to said rotational inertia mass via oneof said second plurality of support arms.
 4. The angular accelerometeras defined in claim 1, wherein said first and second plurality ofsupport arms each comprises a radial extending arm.
 5. The angularaccelerometer as defined in claim 1, wherein each of said secondplurality of support arms comprises a folded semi-circular portion. 6.The angular accelerometer as defined in claim 1, wherein said first andsecond plurality of support arms are substantially equiangularlylocated.
 7. The angular accelerometer as defined in claim 1, whereinsaid rotational inertia mass is substantially centrally located, andsaid first fixed electrode is radially outward from said rotationalinertia mass.
 8. The angular accelerometer as defined in claim 1,wherein said substrate comprises a silicon substrate.
 9. The angularaccelerometer as defined in claim 1, wherein said angular accelerometeris fabricated by a trench etching process.
 10. The angular accelerometeras defined in claim 1, wherein said first and second plurality ofsupport arms each comprises at least four equiangularly located supportarms.
 11. The angular accelerometer as defined in claim 1, wherein eachof said first and second plurality of support arms are flexible so as tobend during angular acceleration about the sensing axis, and yet rigidto resist bending due to acceleration about non-sensing axes.
 12. Anangular accelerometer comprising: a substrate; a first bank of variablecapacitors formed of a first plurality of fixed capacitive plates and afirst plurality of movable capacitive plates; a second bank of variablecapacitors formed of a second plurality of fixed capacitive plates and asecond plurality of movable capacitive plates; a rotational inertia massconfigured as a ring having a central opening and rotatable in responseto angular acceleration and electrically coupled to said first andsecond plurality of movable capacitive plates and arranged so that saidfirst and second plurality of movable capacitive plates form capacitivecouplings with said first and second plurality of fixed capacitiveplates; a central member fixed to said substrate and centrally locatedwithin said central opening of the rotational inertia mass and separatedfrom the rotational inertia mass; an outer member supported on thesubstrate and located radially outward from the rotational inertia mass;a first plurality of support arms extending between said central memberand said rotational inertia mass; and a second plurality of support armsextending between said rotational inertia mass and said outer member,wherein the first and second plurality of support arms support saidrotational inertia mass and movable capacitive plates relative to saidfirst and second fixed capacitive plates and further allow rotationalmovement of the rotational inertia mass upon experiencing an angularacceleration about a sensing axis.
 13. The angular accelerometer asdefined in claim 12 further comprising: a first input electricallycoupled to said first plurality of fixed capacitive plates; a secondinput electrically coupled to said second plurality of fixed capacitiveplates; and an output electrically coupled to said rotational inertiamass for sensing an output signal indicative of angular acceleration inresponse to rotation of said rotational inertia mass.
 14. The angularaccelerometer as defined in claim 13, wherein said output iselectrically coupled to said rotational inertia mass via one of saidsecond plurality of support arms.
 15. The angular accelerometer asdefined in claim 12, wherein said first and second plurality of supportarms each comprises a radial extending arm.
 16. The angularaccelerometer as defined in claim 12, wherein each of said secondplurality of support arms comprises a folded semi-circular portion. 17.The angular accelerometer as defined in claim 12, wherein said first andsecond plurality of support arms are substantially equiangularlylocated.
 18. The angular accelerometer as defined in claim 12, whereinsaid substrate comprises a silicon substrate.
 19. The angularaccelerometer as defined in claim 12, wherein said angular accelerometeris fabricated by a trench etching process.
 20. The angular accelerometeras defined in claim 12, wherein said first and second plurality ofsupport arms each comprises at least four equiangularly located supportarms.
 21. A micromachined angular accelerometer comprising: a substrate;a fixed electrode supported on the substrate and including a firstplurality of fixed capacitive plates; a rotational ring having a centralopening and including a plurality of movable capacitive plates at theouter perimeter and arranged to provide a capacitive coupling with thefirst plurality of fixed capacitive plates, said rotational ring beingsuspended over a cavity and rotationally movable relative to said fixedelectrode; a central member fixed to said substrate and located withinthe central opening of the rotational ring; an outer member fixed tosaid substrate and located radially outward from said rotational ring; afirst plurality of support arms extending between said central memberand the rotational ring; a second plurality of support arms extendingbetween said rotational ring and said outer member, wherein the firstand second plurality of support arms support said rotational ringrelative to said fixed electrode and allow rotational movement of therotational ring upon experiencing an angular acceleration about asensing axis; an input electrically coupled to one of the fixedelectrode and the rotational ring for receiving an input signal; and anoutput electrically coupled to the other of the fixed electrode and therotational ring for providing an output signal which varies as afunction of change in the capacitive coupling and is indicative ofangular acceleration.
 22. The angular accelerometer as defined in claim21, wherein one of the input and output is electrically coupled to therotational ring via one of the second plurality of support arms.
 23. Theangular accelerometer as defined in claim 21, wherein each of said firstand second plurality of support arms comprises an integrally formedsupport arm formed by removing material on opposite sides of saidsupport arm.
 24. The angular accelerometer as defined in claim 21,wherein each of said second plurality of support arms comprises a foldedsemi-circular portion.
 25. The angular accelerometer as defined in claim21, wherein said first and second plurality of support arms eachcomprises a radial extending support arm.
 26. The angular accelerometeras defined in claim 21, wherein each of said first and second pluralityof support arms comprises two pairs of oppositely opposed support arms,wherein said two pairs are arranged approximately ninety degreesrelative to one another.